The present invention relates to a semiconductor laser device suitable for constructing an optical pickup reading information from and writing information to different types of optical discs.
Optical discs are used not only for video/audio recording but also as computer external storage media because they have a large recording capacity and a random access characteristic that the access time is not varied irrespective of an information-recording position. However, different types of optical discs are used according to purpose. Thus, characteristics demanded of the optical pickups differ depending on the type of optical discs for which the optical pickups are used.
In optical pickups reading compact discs (CD) that are used to record audio information and CD-ROM media formed by formatting the CD as computer external storage media, an optical spot having a large diameter can be used. Thus, an infrared semiconductor laser device emitting a light beam having a wavelength in the vicinity of 780 nm is used as a light source, and a condenser lens having a numerical aperture (NA) of about 0.45 is used.
In optical pickups which read information from a digital versatile disc (DVD) having a large memory capacity and used to record video information or the like, it is necessary to make the optical spot smaller. Thus, as the light source of the DVD optical pickup, a red semiconductor laser device emitting a light beam having a wavelength of 630-680 nm is used, and a condenser lens having a NA of about 0.6 is used. The optical pickup for DVD can also read information from the CD and the CD-ROM by adjusting the NA of the condenser lens by, for example, using a diaphragm.
Optical discs called xe2x80x9cCD-Rxe2x80x9d which have an information recording format common to the CD and to which information can be written only once are spreading widely. This is because the optical discs CD-R are not only readable by an ordinary optical pickup for the CD, but also inexpensive. As the material of the CD-R, in consideration of compatibility with the CD, a recording film made of an organic material is used which is designed such that read and write of information can be preferably accomplished for light having a wavelength in the neighborhood of 780 nm. Thus, the CD-R disc is not readable unless an optical pickup using an infrared laser as its light source is employed.
In order for one optical pickup to read information of different types of optical discs such as CD, CD-ROM, CD-R, and DVD, a red semiconductor laser and an infrared semiconductor laser are required as its light source.
Changing the optical pickup for each type of the optical discs causes inconvenience and the device size will become large. Thus, there is a growing demand for the development of an optical pickup that is capable of reading information from and writing information to different types of optical discs, has a size not different from that of the conventional optical pickup for CD, and can be produced with a technique equivalent to that employed to manufacture the conventional optical pickup for CD.
To allow the optical pickup to read information from and write information to different types of optical discs, two kinds of laser light sources, namely, the red semiconductor laser and the infrared semiconductor laser are required, as described above. The sizes of the laser light sources define the size of the optical pickup. That is, a conventional semiconductor laser device has an internal construction shown, for example, in FIG. 6.
In FIG. 6, a semiconductor laser element 3 is fixed to a heat sink block 2 formed integrally with a mount 1 made of a metal disc, using an electrically conductive adhesive material or a soldering material (not shown). The semiconductor laser element is typically about 200 xcexcm in width, about 250 xcexcm in length, and about 100 xcexcm in thickness. Because flat planes are formed on the heat sink block 2 by molding, the length of one side of the heat sink block 2 is set to 2 mm or more.
A monitoring photodiode (PD) 6 is provided in a recess 1a of the mount 1 for monitoring the intensity of light emitted from a rear end face of the semiconductor laser element 3. The monitoring PD 6 is fixed by an electrically conductive adhesive material or a metallic soldering material (not shown) applied in advance to a bottom surface of the recess la and to a lower surface of the monitoring PD 6.
To accommodate the semiconductor laser element 3, the heat sink block 2, and the monitoring PD 6 in one package, after they are covered with a cap (not shown) having a laser beam-emitting window, the cap is welded to the mount 1. The welding of the cap is performed in an atmosphere of an inert gas such as nitrogen, argon or the like or dry air so that moisture does not remain in the package. The cap is welded to the mount 1 with no gap therebetween to keep the inside of the package airtight and prevent moisture from penetrating into the package from outside so that semiconductor devices such as the semiconductor laser element 3 and the monitoring PD 6 are prevented from deteriorating over a long period of time.
To electrically connect the semiconductor devices with the outside, there are provided a plurality of lead pins penetrating through the mount 1 in a manner insulated from the mount 1. More specifically, an upper electrode 3a of the semiconductor laser element 3 and a flat portion 51a of a lead pin 51 having a diameter of about 0.2 mm are electrically connected to each other with a gold wire 501. A surface electrode 6a of the monitoring PD 6 and a front end of a lead pin 52 are connected to each other with a gold wire 502. For insulation, the mount 1 and the lead pins 51, 52 are spaced by a gap of 0.1 mm or more and fixed to each other with an insulating material.
The inner diameter of the cap is such that the cap does not contact the lead pins. To keep the inside of the cap airtight, a part of the cap that contacts the mount 1 has a flat surface having a width of about 0.5 mm. Therefore, it is necessary that the mount 1 should have a diameter of more than 3.8 mm.
An optical axis C of the semiconductor laser element 3 passes through an emission point 301 and is perpendicular to a front end surface of the semiconductor laser element 3. If the distance between the optical axis of the semiconductor laser that is used as a light source and the axis of a condenser lens is sufficiently short, preferably less than 80 xcexcm, it is possible to read information from an optical disc. That is, as the distance between the optical axis of the semiconductor laser element and the axis of the condenser lens becomes longer, spherical aberration increases quadrically. As a result, the spot diameter of the condensed light becomes large and thus information cannot be read. For example, in an optical disc system, a tolerable spherical aberration limit is Marechal limit (0.07xcex, where xcex is a wavelength of a laser beam). In the case of the condenser lens used in an ordinary optical pickup, if the distance between the axis of the condenser lens and the optical axis of the semiconductor laser element is about 80 xcexcm, then the spherical aberration exceeds Marechal limit.
If the infrared semiconductor laser element and the red semiconductor laser element are arranged simply side by side to be used as the light sources, the distance L between the optical axis of an infrared laser beam and that of a red laser beam will be more than 3.8 mm because the diameter of the mount 1 is more than 3.8 mm or more, as described above. In this case, the distance between the center axis of the condenser lens and the optical axis of each laser beam exceeds 80 xcexcm without failure. Accordingly, without shifting the condenser lens, it is impossible to read information from and write information to different types of optical discs.
To solve this problem, an optical pickup as shown in FIG. 7 is described in Nikkei Electronics No. 687, page 138, published on Apr. 21, 1997. This optical pickup uses red and infrared semiconductor laser devices having the same construction as that shown in FIG. 6 as light sources 101 and 102. In the optical pickup, a prism 76 is used to make the distance between the optical axes of the laser beams sufficiently small so as to read information from and write information to different optical discs 81 and 82 without moving a condenser lens 71.
The operation of the optical pickup of FIG. 7 is described below in detail. In the optical pickup, to utilize light with increased efficiency, a polarizing beam splitter is used as the prism 76. The polarization direction of the infrared laser beam and that of the red laser beam are orthogonal to each other. The polarizing beam splitter completely transmits or reflects laser beams emitted by the semiconductor laser devices. These two light beams pass through a xc2xc wavelength plate 72, whereby they are converted into circularly polarized light beams such that they rotate in opposite directions. The circularly polarized light beams are then reflected by the optical disc 81 or 82 and enter the xc2xc wavelength plate 72 again, whereby they are converted into linearly polarized light beams orthogonal to each other. Then, the polarization beam splitter 76 reflects the infrared light completely and transmits the red light completely. Then, the infrared light and the red light return to the respective semiconductor laser devices and detected.
A collimator lens 75 has a function of converting light beams emitted by the semiconductor laser devices 101 and 102 into parallel light beams. Omission of the collimator lens 75 is no problem theoretically. Practically, however, provision of the collimator lens 75 can stabilize the characteristic of the optical pickup.
A polarizing hologram 73 enlarges the diameter of the red laser beam emitted by the semiconductor laser device 102 to thereby allow the effective NA of the condenser lens 71 to be large so that an optical spot of the condensed light beam has a size suitable for reading information of the DVD. Because the polarizing hologram 73 does not have any influence on the infrared light beam, which is orthogonal to the red light beam in the polarization direction, the effective NA of the condenser lens 71 is small for the infrared light beam. Therefore, the spot diameter of the condensed infrared light beam is appropriate to read information from a CD.
A hologram element 77 mounted on the infrared semiconductor laser device 101 has a function of generating track-controlling three light beams and converting the direction of a signal beam which has been reflected by the optical disc 81 so that the signal beam is incident on a photodiode (not shown) for the signal.
The optical pickup is assembled in the following procedure. Optical elements, namely, the prism 76, the collimator lens 75, a rise mirror 74, the polarizing hologram 73, and the xc2xc wavelength plate 72 are positioned and mounted at predetermined positions with respect to the center axis O of the condenser lens 71. The red semiconductor laser device 102 is mounted in such a manner that its optical axis B is substantially coincident with the center axis O of the condenser lens 71. Then, the path of the optical axis A of the infrared semiconductor laser device 101 is changed by 90xc2x0 by the prism 76 and then adjusted such that the optical axis A is parallel to the center axis O of the condenser lens 71. Thereafter, a parallel movement of the infrared semiconductor laser device 101 is performed to make the optical axis A and the axis O of the condenser lens 71 substantially coincident with each other.
If the condenser lens 71 is regarded as a reference, position adjustment of the optical component parts such as the prism 76, the red semiconductor laser device 102, and the infrared semiconductor laser device 101 is required. If deviations of these optical elements from an ideal position are taken as s1, s2, and s3, respectively, then a mechanical mounting accuracy S is expressed by an equation (1) below:
S={square root over (s12+s22+s32)}xe2x80x83xe2x80x83(1)
With the increase of the number of component parts, the mounting accuracy S has a large value, which makes it difficult to assemble them. As is obvious from the above, it is much more difficult to produce an optical pickup capable of reading information from and writing information to different types of optical discs by using the conventional semiconductor laser elements shown in FIG. 6 as its light sources than to produce the conventional optical pickup for CDs that needs no prism.
The present invention has been made in view of the problem, and it is an object of the present invention to provide a semiconductor laser device that enables realization of an optical pickup capable of reading information from and writing information to optical discs of different types, such as CD, CD-R, and DVD, without increasing the number of component parts and with an assembling technique similar to that employed for the conventional optical pickup.
In order to accomplish the above object, the present invention provides a semiconductor laser device for use in an optical pickup reading and writing information by selecting one of light beams emitted by two semiconductor laser elements having different emission wavelengths and then condensing the selected light beam on one of different optical discs through an optical system including a condenser lens, comprising:
a heat sink block; and
a first semiconductor laser element and a second semiconductor laser element having different emission wavelengths and mounted on the heat sink such that optical axes of the first and second semiconductor laser elements are substantially parallel to each other,
wherein the first and second semiconductor laser elements are mounted on the heat sink block in such a manner that the following relationship is satisfied:
0xe2x89xa6Lxe2x89xa6d1+d2xe2x89xa6160 xcexcm,
where d1 is a distance between the optical axis of the first semiconductor laser element and a center axis of the condenser lens, d2 is a distance between the optical axis of the second semiconductor laser element and the center axis of the condenser lens, and L is a distance between the optical axes of the first and second semiconductor laser elements.
The reason the distance L between the optical axes of the first and second semiconductor laser elements is preferably 160 xcexcm or less is as follows:
It is ideal that the axis of the condenser lens is coincident with the optical axis of the semiconductor laser element. That is, it is ideal that the emission point of the semiconductor laser element is present on the axis of the condenser lens. If the axis of the condenser lens is not coincident with the optical axis of the semiconductor laser element, spherical aberration increases quadrically, as described above. As a result, the diameter of an optical spot of condensed light beam becomes large and thus information cannot be read. For example, in an optical disc system, Marechal limit (0.07xcex, where xcex is a wavelength of a laser beam) is a tolerable spherical aberration limit. In a condenser lens for use in an ordinary optical pickup, if the distance between the center axis of the condenser lens and the optical axis of the semiconductor laser element is more than 80 xcexcm, the spherical aberration exceeds Marechal limit. If distances between the axis of the condenser lens and each of optical axes of different two semiconductor laser elements are taken as d1 and d2, respectively, then, neither d1 nor d2 should exceed 80 xcexcm. This condition can be satisfied if the distance L between the optical axes of these two semiconductor laser elements is 160 xcexcm and if the axis of the condenser lens falls on a line connecting the emission points of the two semiconductor laser elements. If L is less than 160 xcexcm, the above condition can be satisfied even though the axis of the condenser lens runs outside the line connecting the emission points of the two semiconductor laser elements. On the other hand, if L is more than 160 xcexcm, at least one of d1 and d2 exceeds 80 xcexcm, irrespective of the position of the axis of the condenser lens. From this, 160 xcexcm is critical as a value of L.
The first semiconductor laser element is for example an infrared laser element and the second semiconductor laser element is for example a red laser element. Of course, it is possible to use semiconductor laser elements of other wavelengths such as a blue laser element.
In the semiconductor laser device having the above construction, the semiconductor laser elements are mounted on the same heat sink block and a relationship of 0xe2x89xa6Lxe2x89xa6d1+d2 less than 160 xcexcm holds between the condenser lens and each of the first and second semiconductor laser elements. Therefore, each of d1 and d2 can be set to 80 xcexcm or less. Accordingly, using this semiconductor laser device, it is possible to produce an optical pickup capable of reading information from and writing information to different types of optical discs without moving or shifting the condenser lens nor increasing the number of component parts.
More specifically, an optical pickup incorporating the semiconductor laser device having the above construction can generate light beams having a wavelength 650 nm and 780 nm along optical paths parallel to each other and spaced at an interval of 160 xcexcm or less. Accordingly, the optical pickup can read information from discs of DVD and CD media including CD-R media currently on the market.
In one embodiment, one of the first and second semiconductor laser elements has been shifted in a direction of its optical axis to rearward of the other semiconductor laser element such that emission end surfaces of the first and second semiconductor laser elements are contained in different planes, and the one semiconductor laser element has been shifted in a direction perpendicular to its optical axis toward the other semiconductor laser element to an extent that the optical axis of the one semiconductor laser element does not pass the other semiconductor laser element.
With this arrangement, it is possible to set the distance L between the two optical axes to 160 xcexcm or less even if a semiconductor laser element having its emission point at its center and having a normal width (200-300 xcexcm) is used as the first/second semiconductor laser element.
In this embodiment, in order to prevent the light emitted from the one semiconductor laser element, namely, the semiconductor laser element located at the rear side in the direction of the optical axis, from being reflected by the heat sink block positioned forward of the semiconductor laser element, it is permissible to cut out a part of the heat sink block located at the front side in the direction in which the outgoing light of the semiconductor laser element advances. Alternatively, the one semiconductor laser element may be mounted on the heat sink block through a sub-mount.
The first and second semiconductor laser elements and the heat sink block are accommodated in a same package.
For the first and second semiconductor laser elements, semiconductor laser elements having emission points at different positions may be used. In this case, the method of manufacturing the first and second semiconductor laser elements is not limited to a specific one, and any emission wavelength can be obtained as desired.
The first and second semiconductor laser elements may be mounted directly on the heat sink block. Alternatively, at least one of the first and second semiconductor laser elements may be mounted on the heat sink block through a sub-mount.
When the sub-mount is used, and if the emission points of the two semiconductor laser elements are at levels different from each other, it is possible to locate these emission points at the same level by adjusting the height of the sub-mount. Further, the semiconductor laser element can be inspected before it is assembled in the semiconductor laser device. Further, it is possible to avoid a possible problem that a soldering material that is used to fix the semiconductor laser element to the heat sink block rises up to the emission point and thus blocks the path of the laser beam partly or the soldering material climbs the semiconductor laser element and thus causes a short circuit.
In one embodiment, the emission points of the first and the second semiconductor laser elements are located between a center axis of the first semiconductor laser element and a center axis of the second semiconductor laser element.
In this case, the distance L between the optical axes can be set to 160 xcexcm or less, irrespective of the widths of the semiconductor laser elements. Therefore, even if a semiconductor laser element having a normal width (200-300 xcexcm) is used as the first/second semiconductor laser element, the distance L between the optical axes can be set to 160 xcexcm or less.
The semiconductor laser device may further comprise a third semiconductor laser element having an emission wavelength different from the wavelengths of the first and second semiconductor laser elements and mounted on the heat sink block such that an optical axis of the third semiconductor laser element is substantially parallel to each of the optical axes of the first and second semiconductor laser elements. In this case, the three semiconductor laser elements are mounted on the heat sink block in such a manner that the following relationships are satisfied:
0xe2x89xa6L1xe2x89xa6d1+d3xe2x89xa6160 xcexcm,
and
0xe2x89xa6L2xe2x89xa6d2+d3xe2x89xa6160 xcexcm,
where d3 is a distance between the optical axis of the third semiconductor laser element and the axis of the condenser lens, L1 is a distance between the optical axes of the first and third semiconductor laser elements, and L2 is a distance between the optical axes of the second and third semiconductor laser elements.
Because the three semiconductor laser elements are mounted on the common heat sink block, relative positions between each two semiconductor laser elements can be set with accuracy.